IELTS

Hi Saurabh,
Some of the popularly followed books include:
1. Target Band 7.
2. IELTS Academic by Simone Braverman.
3. The Official Cambridge Guide to IELTS.
4. New Insight into IELTS.
5. ETS: Official Guide to the TOEFL.
6. Kaplan TOEFL iBT Premier and
7. ETS: Official TOEFL iBT tests.

Dear Vaishnavi,
You can refer to similar queries on IELTS preparation, here are a few links to help you:
https://ask.shiksha.com/how-should-prepare-for-ielts-exam-qna-3541321
https://ask.shiksha.com/how-to-prepare-for-ielts-test-qna-2717659

Please share more information such as your education background and what you plan to study.

Hi Junaid,
Admissions are not done on the basis of IELTS score alone. In fact in Germany, very high academic grades are required. I am afraid with the academic record you have possibilities are very less but you can always try. Keep looking at the other options like UK, Australia as well.

Hi Sindhuja,
This is not explicitly mentioned on their website however they did mention the fact that ' The average GRE score of graduate students in engineering programs is around 1100 on the math and verbal sections, when combined'. Secondly, you must submit a TOEFL score and not IELTS.
I hope this helped.

Four to six months if you have studied in English medium school and speak well. Take a mock test and see if how much preparation is required and join coaching accordingly.

Hi Jayesh,
GMAT is optional and for IELTS you must obtaining an overall minimum score of 6.5 with each paper having 5.5. Please read more here:
http://courses.cardiff.ac.uk/postgraduate/course/detail/p032.html

IELTS is conducted by British Council so most of the German universities prefer IELTS.